This disclosure relates to graphene and the production of graphene, including an apparatus and a method for expansion of graphite to graphene.
Idealized graphene is a one-atom-thick layer of graphite that is infinitely large and impurity free. In the real world, graphene is of finite size and includes impurities. Notwithstanding these imperfections, the physical properties of real-world graphene are dominated by sp2-hybridized carbon atoms that are surrounded by three other carbon atoms disposed in a plane at angles of 120° from one another, thereby approximating an infinite sheet of pure carbon. As a result of this structure, graphene has a number of very unusual physical properties, including very high elastic modulus-to-weight ratios, high thermal and electrical conductivity, and a large and nonlinear diamagnetism. Because of these unusual physical properties, graphene can be used in a variety of different applications, including conductive inks that can be used to prepare conductive coatings, printed electronics, or conductive contacts for solar cells, capacitors, batteries, and the like.
Although idealized graphene includes only a single layer of carbon atoms, graphene structures that include multiple carbon layers (e.g., up to 10 layers, or up to 6 layers) can provide comparable physical properties and can be used effectively in many of these same applications. For the sake of convenience, both single atomic layer graphene and such multi-layered structures with comparable physical properties are referred to as “graphene” herein.
There are a variety of different types of graphene and other carbonaceous flake materials. Basic characteristics of some of these materials are now described.
Chemical Vapor Deposition: Chemical vapor deposition (CVD) can be used to produce graphene monolayers that have large flake sizes and low defect densities. In some cases, CVD yields graphene with multiple layers. In some cases, CVD can yield graphene that has macroscopic flake sizes (e.g., approaching 1 cm in length).
Examples of the use of CVD to produce graphene can be found in Science 342: 6159, p. 720-723 (2013), Science 344: 6181, p. 286-289 (2014), and Scientific Reports 3, Art. No.: 2465 (2013). According to the abstract of this last example, “[c]hemical vapor deposition of graphene on transition metals has been considered as a major step towards commercial realization of graphene. However, fabrication based on transition metals involves an inevitable transfer step which can be as complicated as the deposition of graphene itself.”
Natural graphite: Graphite occurs in nature and can be found in crystalline flake-like form that includes several tens to thousands of layers. The layers are typically in an ordered sequence, namely, the so-called “AB stacking,” where half of the atoms of each layer lie precisely above or below the center of a six-atom ring in the immediately adjacent layers. Because graphite flakes are so “thick,” they display physical properties that differ from those of graphene and many of these physical properties are relevant to different applications. For example, graphite flakes are very weak in shear (i.e., the layers can be separated mechanically) and have highly anisotropic electronic, acoustic, and thermal properties. Due to the electronic interaction between neighboring layers, the electrical and thermal conductivity of graphite is lower than the electrical and thermal conductivity of graphene. The specific surface area is also much lower, as would be expected from a material with a less planar geometry. Further, in typical flake thicknesses, graphite is not transparent to electromagnetic radiation at a variety of different wavelengths. In some cases, graphite flakes can have macroscopic flake sizes (e.g., 1 cm in length).
An example of a characterization of graphite-based systems by Raman spectroscopy can be found in Phys. Chem. Chem. Phys. 9, p. 1276-1290 (2007).
Graphene Oxide: Chemical or electrochemical oxidation of graphite to graphite oxide followed by exfoliation can be used to produce graphene oxide flakes. One of the more common approaches was first described by Hummers et al. in 1958 and is commonly referred to as “Hummer's method.” J. Am. Chem. Soc. 80 (6) p. 1339-1339 (1958). In some cases, the graphene oxide can subsequently be partially reduced to remove some of the oxygen.
However, oxidative etching of graphite not only separates graphene layers from each other, but also attacks the hexagonal graphene lattice. In general, the resulting graphene oxide is defect-rich and, as a result, displays reduced electrical- and thermal-conductivity, as well as a reduced elastic modulus. In addition, the in-plane etching of graphene flakes typically leads to relatively smaller lateral dimensions, with flake sizes being below few micrometers. In some cases, the average size of graphene oxide flakes in a polydisperse sample can be increased using physical methods such as, e.g., centrifugation.
Examples of methods for producing and/or handling graphene oxide can be found in Carbon 50(2) p. 470-475 (2012) and Carbon 101 p. 120-128 (2016).
Liquid phase exfoliation: Flakes of carbonaceous material can be exfoliated from graphite in a suitable chemical environment (e.g., in an organic solvent or in a mixture of water and surfactant). The exfoliation is generally driven by mechanical force provided by, e.g., ultrasound or a blender. Examples of methods for liquid phase exfoliation can be found in Nature Materials 13 p. 624-630 (2014) and Nature Nanotechnology 3, p. 563 -568 (2008).
Although the researchers who work with liquid phase exfoliation techniques often refer to the exfoliated carbonaceous flakes as “graphene,” the thickness of the vast majority of flakes produced by such exfoliation techniques often appears to be in excess of 10 layers. This can be confirmed using, e.g., Raman spectroscopc techniques. For example, in Phys. Rev. Lett. 2006, 97, 187401, an asymmetric shape of the Raman band around 2700 reciprocal centimeters indicates that these flakes are thicker than 10 layers. Indeed, the predominant thickness of such flakes often appears to be in excess of 100 layers, which can be confirmed by x-ray diffraction, scanning probe microscopy or scanning electron microscopy. As a result of this large thickness, the material properties often do not correspond to the properties expected from graphene. At 10 layers, properties like thermal conductivity approach the values of bulk graphite with AB stacking, as described in Nat. Mater. 2010, 9, 555-558. Properties like the specific surface area also scale with the inverse of the flake thickness.
Exfoliation of expanded graphite: Graphite can be expanded using thermal techniques such as, e.g., microwave irradiation. Flakes of carbonaceous material can be exfoliated from the expanded graphite in a suitable chemical environment (e.g., in an organic solvent or in a mixture of water and surfactant). The exfoliation is generally driven by mechanical force such as, e.g., ultrasound or a shear force from a blender. Examples of methods for liquid phase exfoliation of expanded graphite can be found in J. Mater. Chem. 22 p. 4806-4810 (2012) and WO 2015131933 A1.
Although the researchers who work with exfoliation of expanded graphite often refer to the exfoliated carbonaceous flakes as “graphene,” the thickness of most of these flakes also appears to be in excess of 10 layers and even in excess of 100 layers. Analytical techniques for determining the thickness of flakes exfoliated from expanded graphite—and the consequences of this thickness—are similar to those described above with respect to liquid phase exfoliation.
Reduction of graphite: Graphite can be reduced and graphene exfoliated in strongly reductive environments via, e.g., Birch reduction in lithium. As graphene is increasingly reduced, more and more carbon atoms become hydrogenated and sp3-hybridized. In theory, the atomic C/H ratio can approach one, i.e., the resulting material becomes graphane rather than graphene. Examples of methods for the reduction of graphite can be found in J. Am. Chem. Soc. 134, p. 18689-18694 (2012) and Angew. Chem. Int. Ed. 52, p. 754-757 (2013).
Lithium and other reductants that can be used to reduce graphite are very strong, difficult to handle, and difficult to dispose.
Electrochemical expansion: Graphene can also be produced by electrochemical cathodic treatment. Examples of methods for electrochemical expansion can be found in WO2012120264 A1 and J. Am. Chem. Soc. 133, p. 8888-8891 (2011). The reductive environment can also induce hydrogenation of the resulting flakes, as described in Carbon 83, p. 128-135 (2015) and WO2015019093 A1. In general, electrochemical expansion at conventional conditions often cannot produce a significant amount of graphene flakes with a thickness below 10 layers, which can be confirmed using Raman spectroscopy.
For the sake of validating the various analytical techniques described herein, various materials have been used as references.
A first such reference material is reduced graphene oxide obtained from Graphenea S. A. (Avenida Tolosa 76,20018—San Sebastián SPAIN.) According to Graphenea S. A.'s product datasheet (available at https://cdn.shopify.com/s/files/1/0191/2296/files/Graphenea_rGO_Datasheet_2014-03-25.pdf?2923), this sample is 77-87 atomic % carbon, 0-1 atomic % hydrogen, 0-1 atomic % nitrogen, 0 atomic % sulfur, and 13-22 atomic % oxygen. It is believed that the reduced graphene oxide in this sample was produced by a modified Hummer's and subsequent chemical reduction. For the sake of convenience, this material is referred to as “GRAPHENEA RGO” herein.
A second such reference material was obtained from Thomas Swan & Co. Ltd. (Rotary Way, Consett, County Durham, DH8 7ND, United Kingdom) under the trademark “ELICARB GRAPHENE.” The datasheet for this material is available at http://www.thomas-swan.co.uk/advanced-materials/elicarb%C2%AE-graphene-products/elicarb%C2%AE-graphene. According to this datasheet, the graphene in this sample was produced by solvent exfoliation and particle size is in the 0.5 to 2.0 micrometer range. For the sake of convenience, this material is referred to as “ELICARB GRAPHENE” herein.
A third such reference material is expanded graphite (EG), produced by thermal expansion of conventional graphite intercalation compounds that are typically produced by chemical oxidation. One example expanded graphite is “L2136,” a non-commercial material made available by Schunk Hoffmann Carbon Technologies AG (Au 62, 4823 Bad Goisern am Hallstättersee, Austria). The company does not disclose details about the manufacturing at the present time. For the sake of convenience, this material is referred to as “L2136” herein.